In the conventional OVD method, as shown in Fig. 1, fine glass particles are deposited on a seed rod 1 by jetting a gaseous glass raw material and a fuel gas from a burner 2 on the rod from a direction perpendicular to a longitudinal axis of the seed rod 1 while rotating the seed rod around its axis and reciprocally moving the seed rod 1 or the burner 2 in the axial direction of the seed rod 1 to form a soot rod 3 having a predetermined length and a symmetrical shape around the axis.
However, according the conventional OVD method, bulk density of the glass preform fluctuates in its radial direction, which results in cracking of the produced soot and fluctuation of concentration of an additive (eg. fluorine) in the radial direction, which additive is added when the soot is sintered. The reasons for these drawbacks are as follows:
In the conventional OVD method, a distance between the burner and the axis of the seed rod is fixed and the jetting rates of the glass raw material and the fuel gas are kept constant. The temperature of a part of the soot rod on which the fine glass particles are being deposited varies with the growth of the soot rod because the distance between the burner and said part of the soot rod becomes closer, and heat capacity of the soot rod and cooling efficiency vary. The variation of the temperature of said part of the soot rod results in the fluctuation of the bulk density of the soot rod in its radial direction, which leads to the soot cracking, the fluctuation of the additive concentration in its radial direction and/or formation of bubbles in the soot rod.